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Making and restoring backups

To begin with: nbackup.exe is located in the /bin subdirectory of your Firebird folder. Typical locations are e.g. C:\Program Files\Firebird\Firebird_2_0\bin (Windows) or /opt/firebird/bin (Linux). Just like most of the tools that come with Firebird, nbackup has no graphical interface; you launch it from the command prompt or from within a batch file or application.

Warning: Under heavy-load circumstances in some environments, nbackup 2.0.3 and below may cause problems that will lead to deadlocks or even corrupted databases. While these problems aren't common, they are serious enough to warrant upgrading to Firebird 2.0.4 or higher if you want to use nbackup comfortably. If it concerns large databases under Posix, the use of direct I/O may also make a difference. More about this in the section Direct I/O.

Full backups

Making full backups

To make a full database backup, the command syntax is:

 nbackup [-U <user> -P <password>] -B 0 <database> [<backupfile>]

For instance:

 C:\Data> nbackup -B 0 inventory.fdb inventory_1-Mar-2006.nbk

Comments:

• The parameter -B stands for backup (gee!). The backup level 0 indicates a full backup. Backup levels greater than 0 are used for incremental backups; we'll discuss those later on.
• Instead of a database filename you may also specify an alias.
• Instead of a backup filename you may also specify stdout. This will send the backup to standard output, from where you can redirect it to e.g. a tape archiver or a compression tool.
• The -U (user) and -P (password) parameters may be omitted if at least one of the following conditions is met:
  • The environment variables ISC_USER and ISC_PASSWORD have been set, either to SYSDBA or to the owner of the database.
  • You are logged on as root on a Posix system. This makes you SYSDBA by default.
  • Under Windows: Trusted authentication is enabled in firebird.conf, and you are logged on to the Windows account that owns the database. This is possible in Firebird 2.1 and above.
  • Under Windows: Trusted authentication is enabled in firebird.conf, and you are logged on as a Windows administrator. In Firebird 2.1, this automatically gives you SYSDBA rights. In Firebird 2.5 and above, there is the additional condition that AUTO ADMIN MAPPING has been set in the database.

• Starting with Firebird 2.5, instead of -P <password> you may also use -FE <filename>. This will cause nbackup to fetch the password from the given file. With -FE, the password itself doesn't appear in the command and will thus be better shielded against people who might otherwise pick it up via the command history, the w command on Unix or from a script or batch file.
• In Firebird 2.1 and up, the firing of database triggers can be prevented by specifying the -T option. For more information, see Suppressing database triggers.
• Starting with Firebird 2.1.4, it is possible to force direct I/O on or off by specifying -D on or -D off. For details and background see Direct I/O, elsewhere in this manual.
• The different parameters (-B, -U etc.) may occur in any order. Of course each parameter should be immediately followed by its own argument(s). In the case of -B there are three of them: backup level, database, and backup file - in that order!
• If the -B parameter comes last, you may leave out the name of the backup file. In that case nbackup will compose a filename based on the database name, the backup level, and the current date and time. This can lead to a name clash (and a failed backup) if two backup commands of the same level are issued in the same minute.

Warning: Do not use nbackup for multi-file databases. This can lead to corruption and loss of data, despite the fact that nbackup will not complain about such a command.

A word on the inner workings

Note: What follows here is not necessary knowledge to use nbackup. It just gives a rough (and incomplete) impression of what happens under the hood during execution of nbackup -B:

1. First of all, the main database file is locked by changing an internal state flag. From this moment on, any and all mutations in the database are written to a temporary file - the difference file or delta file.

2. Then the actual backup is made. This isn't a straight file copy; restoring must be done by nbackup as well.

3. Upon completion of the backup, the contents of the delta file are integrated with the main database file. After that, the database is unlocked (flag goes back to "normal") and the delta is removed.

The functionality of steps 1 and 3 is provided by two new SQL statements: ALTER DATABASE BEGIN BACKUP and ALTER DATABASE END BACKUP. Contrary to what the names suggest, these statements do not take care of making the actual backup; rather, they create the conditions under which the main database file can be safely backed up. And to be clear: you don't need to issue these commands yourself; nbackup will do that for you, at the right moments.

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Restoring a full backup

A full backup is restored as follows:

 nbackup [-U <user> -P <password>] -R <database> [<backupfile>]

For instance:

 C:\Data> nbackup -R inventory.fdb inventory_1-Mar-2006.nbk

Comments:

• You don't specify a level for a restore.
• When restoring, the -R parameter must come last, for reasons that will become clear later.
• Instead of a database filename you may also use an alias.
• If the specified database file already exists, the restore fails and you get an error message.
• Here too, you may omit the name of the backup file. If you do, nbackup will prompt you for it. (Attention! In Firebird 2.0 this “interactive restore” feature is broken, leaving you with an error message and a failed restore. Fixed in 2.0.1.)
• Restoring works purely on the filesystem level and can even be done without a Firebird server running. Any credentials supplied via the -U and -P parameters are ignored. The same goes for passwords read from a file. However, nbackup does try to read the password from the file if the -FE parameter is present, and if an error occurs, the entire operation is abandoned.

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Incremental backups

Warning: The incremental backup facility is entirely broken in Firebird 2.1, and fixed again in 2.1.1.

Making incremental backups

To make an incremental ("differential") backup we specify a backup level greater than 0. An incremental backup of level N always contains the database mutations since the most recent level N-1 backup.

Examples:

One day after the full backup (level 0), you make one with level 1:

 C:\Data> nbackup -B 1 inventory.fdb inventory_2-Mar-2006.nbk

This backup will only contain the mutations of the last day.

One day later again, you make another one with level 1:

 C:\Data> nbackup -B 1 inventory.fdb inventory_3-Mar-2006.nbk

This one contains the mutations of the last two days, since the full backup, not only those since the previous level 1 backup.

A couple of hours on we go for a level 2 backup:

 C:\Data> nbackup -B 2 inventory.fdb inventory_3-Mar-2006_2.nbk

This youngest backup only contains the mutations since the most recent level 1 backup, that is: of the last few hours.

Note: All the comments that have been made about full backups also apply to incremental backups.

Warning: Again: do not use nbackup for multi-file databases.

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Restoring incremental backups

When restoring incremental backups you must specify the entire chain of backup files, from level 0 through the one you wish to restore. The database is always built up from the ground, step by step. (It is this stepwise adding until the database is restored that gave rise to the term incremental backup.)

The formal syntax is:

 nbackup -R <database> [<backup0> [<backup1> [...] ] ]

So restoring the level 2 backup from the previous example goes as follows:

 C:\Data> nbackup -R inventory.fdb inventory_1-Mar-2006.nbk
          inventory_3-Mar-2006.nbk inventory_3-Mar-2006_2.nbk

Of course the line has been split here for layout reasons only - in reality you type the entire command and only hit [Enter] at the end.

Comments (in addition to the comments with restoring a full backup):

• Because it is not known beforehand how many filenames will follow the -R switch (as we don't specify a level when restoring), nbackup considers all arguments after the -R to be names of backup files. It is for this reason that no other parameter may follow the list of filenames.
• There is no formal limit to the number of backup levels, but in practice it will rarely make sense to go beyond 3 or 4.

Non-connecting links

What happens if you accidentally leave out a file, or specify a series of files that don't all belong together? You could imagine that you specify inventory_2-Mar-2006.nbk by mistake instead of inventory_3-Mar-2006.nbk in the above example. Both are level 1 backup files, so in both cases we get a nice 0, 1, 2 level series.

But our level 2 file is incremental to the level 1 backup of 3 March, not to the one of 2 March.

Fortunately such a mistake can never lead to an incorrectly restored database. Each backup file has its own unique ID. Furthermore, each backup file of level 1 or above contains the ID of the backup on which it is based. When restoring, nbackup checks these IDs; if somewhere in the chain the links don't connect, the operation is cancelled and you get an error message.

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Backing up raw-device databases

Firebird databases need not be files; they can also be placed on a so-called raw device, for instance a disk partition without a file system. The question where the delta has to be placed in such cases was at first overlooked during the development of nbackup. On Posix systems, if the database was located at e.g. /dev/hdb5, it could happen that the delta was created as /dev/hdb5.delta. In light of the nature and purpose of the /dev directory and its often limited available space, this is undesirable.

As of Firebird 2.1, nbackup refuses to operate on raw-device databases unless an explicit location for the delta file has been set. The way to do this is discussed in Setting the delta file, later on in this manual.

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Suppressing database triggers (Firebird 2.1+)

Firebird 2.1 introduced the concept of database triggers. Certain types of these triggers can fire upon making or breaking a database connection. As part of the backup process, nbackup opens a regular connection to the database (in some versions even more than once). To prevent database triggers from firing inadvertently, the new -T switch can be used. Notice that the corresponding switches in gbak and isql are called -nodbtriggers (we love diversity, here at Firebird).

Direct I/O (Firebird 2.1.4+)

Originally, nbackup used direct I/O only when making a backup under Windows NT (and successors like 2000, 2003 etc.) On all other OS'es, direct I/O was off. This caused problems on some Linux systems, so in versions 2.0.6 and 2.1.3 direct I/O was switched on under Linux as well. However, this turned out to be problematic for certain other Linux configurations. In 2.1.4 and 2.5 the original behaviour was restored, but this time as a default that was overridable by a newly added parameter: -D. Its use is as follows:

 nbackup -B 0 cups.fdb cups.nbk -D on -- direct I/O on
 nbackup -B 0 mugs.fdb mugs.nbk -D off -- direct I/O off

Just like the option letters themselves, the arguments ON and OFF are case-insensitive.

Direct I/O is only applied when making a backup, not during a restore. Under Windows it is realized by setting FILE_FLAG_NO_BUFFERING. On other systems, O_DIRECT and POSIX_FADV_NOREUSE are used. The latter two are sometimes unavailable; in such cases, they are (or one of them is) silently left out. Even if the user specified -D on explicitly, this doesn't lead to a warning or error message.

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Informational options (Firebird 2.5+)

Apart from the already mentioned -FE and -D parameters, Firebird 2.5 also saw the introduction of the following two:

-Z
Shows single-line version information. This option can be used independently, but also in combination with other parameters, such as -B, -R, -L etc.

-?
Shows a summary of nbackup's usage and command-line parameters. Attention: If this option is present, all the other parameters are ignored!

Backups on remote machines (Firebird 2.5+)

nbackup itself only operates on local databases. But in Firebird 2.5 and up, nbackup-type backups and restores can also be performed remotely via the Services Manager. For this, the program fbsvcmgr.exe on the local machine is used; it is located in the same folder as nbackup.exe and the other Firebird command-line tools. The first argument is always hostname:service_mgr, with hostname being the name of the remote server. Other available parameters are:

 -user username
 -password password
 -action_nbak
 -action_nrest
 -nbk_level n
 -dbname database
 -nbk_file filename
 -nbk_no_triggers
 -nbk_direct on|off

Making a full backup on the remote machine frodo goes like this:

 fbsvcmgr frodo:service_mgr -user sysdba -password masterke
   -action_nbak -nbk_level 0
   -dbname C:\databases\countries.fdb -nbk_file C:\databases\countries.nbk

And a subsequent incremental backup:

 fbsvcmgr frodo:service_mgr -user sysdba -password masterke
   -action_nbak -nbk_level 1
   -dbname C:\databases\countries.fdb -nbk_file C:\databases\countries_1.nbk

To restore the whole shebang:

 fbsvcmgr frodo:service_mgr -user sysdba -password masterke
   -action_nrest -dbname C:\databases\countries_restored.fdb
   -nbk_file C:\databases\countries.nbk -nbk_file C:\databases\countries_1.nbk

Notice: Each of the above commands should be typed as a single sentence, without line breaks. The hyphens before the parameter names may be omitted, but especially with long commands like these it may be helpful to leave them in, so you can easily identify the individual parameters (the arguments don't get a hyphen).

Comments:

• The Services Manager always requires authentication, be it automatic (root under Posix, trusted under Windows) or explicit through the parameters -user and -password. The environment variables ISC_USER and ISC_PASSWORD are not used. AUTO ADMIN MAPPING in the database has no effect when connecting remotely (though this may also depend on the configuration of the network).

Note: When Windows trusted authentication is in effect, the account name of the user on the local machine is passed to the Services Manager on the remote machine. If the owner of the remote database is a Windows account (e.g. FRODO\PAUL) rather than a Firebird account, and the Windows account name on the local machine is the same as the owner account name on the remote machine, the caller is acknowledged as the database owner and allowed to make a backup. This could pose a security risk, because even on local networks user PAUL on one machine is not necessarily the same person as user PAUL on another machine.

• Restoring (-action_nrest) also requires authentication, but once verified the credentials are not used in any way. Hence, the user need not be the database owner, SYSDBA or superuser. In the case of Windows trusted authentication, the user need not exist at all on the remote machine (where the database is located).

This weak authentication implies another potential security risk. Suppose a sensitive database is nbackupped, and the backups are well protected on the filesystem level. An average user can't restore the database with nbackup then, because nbackup runs in the user process space. But that same user, if he knows name and location of the backup, or can guess them by analogy, might be able to get hold of the database by having fbsvcmgr restore it to a public folder. After all, fbsvcmgr calls the Firebird server, which may have file-level access to the backup. Of course there are solutions to this, but it's important to be aware of the risk.

• The Services Manager can also be used locally; in that case the first argument becomes service_mgr, without hostname. When used locally, AUTO ADMIN MAPPING has the intended effect; this is still true if you prepend localhost: or the name of the local machine. Local use of the Services Manager can be beneficial if you don't have filesystem access to the database and/or backup files, but the Firebird server process does. If you do have sufficient rights, then it's more practical to use nbackup itself, with its much shorter commands.
• Specifying -nbk_no_triggers or -nbk_direct with -action_nrest leads to an error message. nbackup itself is more lenient here: it simply ignores the -T and -D parameters if they are used in the wrong context.
• Instead of a database filename you may also use an alias.

A practical application

An nbackup-based incremental backup scheme could look like this:

• Each month a full backup (level 0) is made;
• Each week a level 1;
• A level 2 backup daily;
• A level 3 backup hourly.

As long as all backups are preserved, you can restore the database to its state at any hour in the past. For each restore action, a maximum of four backup files is used. Of course you schedule things in such a way that the bigger, time-consuming backups are made during off-peak hours. In this case the levels 0 and 1 could be made at weekends, and level 2 at night.

If you don't want to keep everything for eternity, you can add a deletion schedule:

• Level 3 backups are deleted after 8 days;
• Level 2s after a month;
• Level 1s after six months;
• Full backups after two years, but the first one of each year is kept.

This is only an example of course. What's useful in an individual case depends on the application, the size of the database, its activity, etc.

Read on?

At this point you know everything you need in order to make and restore full and/or incremental backups with nbackup. You only need to read any further if you want to use backup tools of your own choice for your Firebird databases (see Locking and unlocking), or if you want to override the default name or location of the delta file (see Setting the delta file).

If you have no craving for any of that: good luck in your work with nbackup!

See also:
InterBase^® and Firebird command-line utilities: nbackup

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